His voice shaking at times, Alex Rodriguez met head-on allegations that he tested positive for steroids six years ago, telling ESPN on Monday that he did take performance-enhancing drugs while playing for the Texas Rangers during a three-year period beginning in 2001.

"When I arrived in Texas in 2001, I felt an enormous amount of pressure. I felt like I had all the weight of the world on top of me and I needed to perform, and perform at a high level every day," Rodriguez told ESPN's Peter Gammons in an exclusive interview in Miami Beach, Fla.

"Back then, [baseball] was a different culture," Rodriguez said. "It was very loose. I was young. I was stupid. I was naive. And I wanted to prove to everyone that I was worth being one of the greatest players of all time.

"I did take a banned substance. And for that, I am very sorry and deeply regretful."

In his first prime-time news conference, President Barack Obama called Rodriguez's admission "depressing" news.

"And if you're a fan of Major League Baseball, I think it tarnishes an entire era, to some degree," Obama said. "And it's unfortunate, because I think there were a lot of ballplayers who played it straight."

Major League Baseball had no comment Monday. Rangers owner Tom Hicks said Rodriguez's admission caught him by surprise.

"I feel personally betrayed. I feel deceived by Alex," Hicks said in a conference call, according to The Associated Press. "He assured me that he had far too much respect for his own body to ever do that to himself. ... I certainly don't believe that if he's now admitting that he started using when he came to the Texas Rangers, why should I believe that it didn't start before he came to the Texas Rangers?"

Rodriguez's admission comes 48 hours after Sports Illustrated reported that Rodriguez was on a list of 104 players who tested positive for banned substances in 2003, the year when Major League Baseball conducted survey tests to see if mandatory, random drug-testing was needed in the sport.

Sources who know about the testing results told SI that Rodriguez tested positive for testosterone and Primobolan, an anabolic steroid. In his ESPN interview, which his ex-wife, Cynthia, attended, Rodriguez said he did not know exactly which substance or substances he had taken. In 2003, there were no penalties for a positive result.

"Again, it was such a loosey-goosey era," Rodriguez said. "I'm guilty for a lot of things. I'm guilty for being negligent, naive, not asking all the right questions. And to be quite honest, I don't know exactly what substance I was guilty of using."

A three-time AL Most Valuable Player, Rodriguez blamed himself and his $252 million contract he signed with the Rangers in 2001 for his decision to use PEDs.

"I felt a tremendous pressure to play, and play really well" in Texas, the New York Yankees third baseman said. "I had just signed this enormous contract ... I felt like I needed something, a push, without over-investigating what I was taking, to get me to the next level."

Rodriguez also said part of the reason he started using drugs was the heat in Texas.

"Can I have an edge just to get out there and play every day?" he said to himself. "You basically end up trusting the wrong people. You end up, you know, not being very careful about what you're ingesting."

Rodriguez added: "I am sorry for my Texas years. I apologize to the fans of Texas."

Rodriguez, who joined the Yankees for the 2004 season after a trade from Texas, said his years as a Yankee "have been clean."

"I've played the best baseball of my career since," he said. "I've won two MVPs since and I've never felt better in my career. Of that I'm very proud of."

The Yankees issued a statement.

"We urged Alex to be completely open, honest and forthcoming in addressing his use of performance-enhancing drugs," it read. "We take him at his word that he was. Although we are disappointed in the mistake he spoke to today, we realize that Alex -- like all of us -- is a human being not immune to fault.

"We speak often about the members of this organization being part of a family, and that is never more true than in times of adversity. Alex took a big step by admitting his mistake, and while there is no condoning the use of performance-enhancing drugs, we respect his decision to take accountability for his actions. We support Alex, and we will do everything we can to help him deal with this challenge and prepare for the upcoming season."

Rodriguez described the last days' turn of events as the biggest challenge of his life but added it felt good to be honest about what he's done in the past.

"It's been a rough 15 months here for me," Rodriguez said.

He added: "I was stupid for three years. I was very, very stupid" and, later said: "The more honest we can all be, the quicker we can get baseball [back] to where it needs to be."

"When you take this gorilla and this monkey off your back, you realize that honesty is the only way," Rodriguez said. "I'm finally beginning to grow up. I'm pretty tired of being stupid and selfish, you know, about myself. The truth needed to come out a long time ago. I'm glad it's coming out today."

He said he stopped using during spring training 2003, when he sustained a neck injury. It was just as baseball started its drug-testing survey. It was only in 2004 that testing with penalties began.

"It wasn't a real dramatic day once I arrived in Texas that something monumental happened in my life," he said. "The point of the matter was that I started experimenting with things that today are not legal or today are not accepted and today you would get in a lot of trouble for."

He said the culture earlier this decade of taking performance-enhancing substances was "prevalent." "There were a lot of people doing a lot of things," Rodriguez said, noting that he wasn't specifically pointing out the Rangers.

"You have nutritionists, you have doctors, you have trainers. That's the right question today: Where did you get it? We're in the era of BALCO," Rodriguez said. "There's many things that you can take that are banned substances. I mean, there's things that have been removed from GNC today that would trigger a positive test."

Rodriguez said he was told by Gene Orza, the chief operating officer of the MLB Players Association, that he might, or might not, have tested positive in the 2003 survey. That conversation happened during the 2004 season. A source told ESPN on Saturday that Rodriguez knew he had failed the test.

According to the Mitchell report, all players who failed the test in 2003 were notified by September 2004.

Rodriguez said he didn't know for sure he had failed a test until Sports Illustrated contacted him last week.